Dissemination of Flow Specification Rules for IPv6
RFC 8956



Internet Engineering Task Force (IETF)                     C. Loibl, Ed.
Request for Comments: 8956                       next layer Telekom GmbH
Updates: 8955                                             R. Raszuk, Ed.
Category: Standards Track                        NTT Network Innovations
ISSN: 2070-1721                                            S. Hares, Ed.
                                                                  Huawei
                                                           December 2020

           Dissemination of Flow Specification Rules for IPv6

Abstract

   "Dissemination of Flow Specification Rules" (RFC 8955) provides a
   Border Gateway Protocol (BGP) extension for the propagation of
   traffic flow information for the purpose of rate limiting or
   filtering IPv4 protocol data packets.

   This document extends RFC 8955 with IPv6 functionality.  It also
   updates RFC 8955 by changing the IANA Flow Spec Component Types
   registry.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8956.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Definitions of Terms Used in This Memo
   2.  IPv6 Flow Specification Encoding in BGP
   3.  IPv6 Flow Specification Components
     3.1.  Type 1 - Destination IPv6 Prefix
     3.2.  Type 2 - Source IPv6 Prefix
     3.3.  Type 3 - Upper-Layer Protocol
     3.4.  Type 7 - ICMPv6 Type
     3.5.  Type 8 - ICMPv6 Code
     3.6.  Type 12 - Fragment
     3.7.  Type 13 - Flow Label (new)
     3.8.  Encoding Examples
   4.  Ordering of Flow Specifications
   5.  Validation Procedure
   6.  IPv6 Traffic Filtering Action Changes
     6.1.  Redirect IPv6 (rt-redirect-ipv6) Type 0x000d
   7.  Security Considerations
   8.  IANA Considerations
     8.1.  Flow Spec IPv6 Component Types
     8.2.  IPv6-Address-Specific Extended Community Flow Spec IPv6
           Actions
   9.  Normative References
   Appendix A.  Example Python Code: flow_rule_cmp_v6
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Introduction

   The growing amount of IPv6 traffic in private and public networks
   requires the extension of tools used in IPv4-only networks to also
   support IPv6 data packets.

   This document analyzes the differences between describing IPv6
   [RFC8200] flows and those of IPv4 packets.  It specifies new Border
   Gateway Protocol [RFC4271] encoding formats to enable "Dissemination
   of Flow Specification Rules" [RFC8955] for IPv6.

   This specification is an extension of the base established in
   [RFC8955].  It only defines the delta changes required to support
   IPv6, while all other definitions and operation mechanisms of
   "Dissemination of Flow Specification Rules" will remain in the main
   specification and will not be repeated here.

1.1.  Definitions of Terms Used in This Memo

   AFI:      Address Family Identifier

   AS:       Autonomous System

   NLRI:     Network Layer Reachability Information

   SAFI:     Subsequent Address Family Identifier

   VRF:      Virtual Routing and Forwarding

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  IPv6 Flow Specification Encoding in BGP

   [RFC8955] defines SAFIs 133 (Dissemination of Flow Specification
   rules) and 134 (L3VPN Dissemination of Flow Specification rules) in
   order to carry the corresponding Flow Specification.

   Implementations wishing to exchange IPv6 Flow Specifications MUST use
   BGP's Capability Advertisement facility to exchange the Multiprotocol
   Extension Capability Code (Code 1), as defined in [RFC4760].  The
   (AFI, SAFI) pair carried in the Multiprotocol Extension Capability
   MUST be (AFI=2, SAFI=133) for IPv6 Flow Specification rules and
   (AFI=2, SAFI=134) for L3VPN Dissemination of Flow Specification
   rules.

3.  IPv6 Flow Specification Components

   The encoding of each of the components begins with a Type field (1
   octet) followed by a variable length parameter.  The following
   sections define component types and parameter encodings for IPv6.

   Types 4 (Port), 5 (Destination Port), 6 (Source Port), 9 (TCP Flags),
   10 (Packet Length), and 11 (DSCP), as defined in [RFC8955], also
   apply to IPv6.  Note that IANA has updated the "Flow Spec Component
   Types" registry in order to contain both IPv4 and IPv6 Flow
   Specification component type numbers in a single registry
   (Section 8).

3.1.  Type 1 - Destination IPv6 Prefix

   Encoding:  <type (1 octet), length (1 octet), offset (1 octet),
      pattern (variable), padding (variable) >

   This defines the destination prefix to match.  The offset has been
   defined to allow for flexible matching to portions of an IPv6 address
   where one is required to skip over the first N bits of the address.
   (These bits skipped are often indicated as "don't care" bits.)  This
   can be especially useful where part of the IPv6 address consists of
   an embedded IPv4 address, and matching needs to happen only on the
   embedded IPv4 address.  The encoded pattern contains enough octets
   for the bits used in matching (length minus offset bits).

   length:    This indicates the N-th most significant bit in the
              address where bitwise pattern matching stops.

   offset:    This indicates the number of most significant address bits
              to skip before bitwise pattern matching starts.

   pattern:   This contains the matching pattern.  The length of the
              pattern is defined by the number of bits needed for
              pattern matching (length minus offset).

   padding:   This contains the minimum number of bits required to pad
              the component to an octet boundary.  Padding bits MUST be
              0 on encoding and MUST be ignored on decoding.

   If length = 0 and offset = 0, this component matches every address;
   otherwise, length MUST be in the range offset < length < 129 or the
   component is malformed.

   Note: This Flow Specification component can be represented by the
   notation ipv6address/length if offset is 0 or ipv6address/offset-
   length.  The ipv6address in this notation is the textual IPv6
   representation of the pattern shifted to the right by the number of
   offset bits.  See also Section 3.8.

3.2.  Type 2 - Source IPv6 Prefix

   Encoding:  <type (1 octet), length (1 octet), offset (1 octet),
      pattern (variable), padding (variable) >

   This defines the source prefix to match.  The length, offset,
   pattern, and padding are the same as in Section 3.1.

3.3.  Type 3 - Upper-Layer Protocol

   Encoding:  <type (1 octet), [numeric_op, value]+>

   This contains a list of {numeric_op, value} pairs that are used to
   match the first Next Header value octet in IPv6 packets that is not
   an extension header and thus indicates that the next item in the
   packet is the corresponding upper-layer header (see Section 4 of
   [RFC8200]).

   This component uses the Numeric Operator (numeric_op) described in
   Section 4.2.1.1 of [RFC8955].  Type 3 component values SHOULD be
   encoded as a single octet (numeric_op len=00).

   Note: While IPv6 allows for more than one Next Header field in the
   packet, the main goal of the Type 3 Flow Specification component is
   to match on the first upper-layer IP protocol value.  Therefore, the
   definition is limited to match only on this specific Next Header
   field in the packet.

3.4.  Type 7 - ICMPv6 Type

   Encoding:  <type (1 octet), [numeric_op, value]+>

   This defines a list of {numeric_op, value} pairs used to match the
   Type field of an ICMPv6 packet (see also Section 2.1 of [RFC4443]).

   This component uses the Numeric Operator (numeric_op) described in
   Section 4.2.1.1 of [RFC8955].  Type 7 component values SHOULD be
   encoded as a single octet (numeric_op len=00).

   In case of the presence of the ICMPv6 type component, only ICMPv6
   packets can match the entire Flow Specification.  The ICMPv6 type
   component, if present, never matches when the packet's upper-layer IP
   protocol value is not 58 (ICMPv6), if the packet is fragmented and
   this is not the first fragment, or if the system is unable to locate
   the transport header.  Different implementations may or may not be
   able to decode the transport header.

3.5.  Type 8 - ICMPv6 Code

   Encoding:  <type (1 octet), [numeric_op, value]+>

   This defines a list of {numeric_op, value} pairs used to match the
   code field of an ICMPv6 packet (see also Section 2.1 of [RFC4443]).

   This component uses the Numeric Operator (numeric_op) described in
   Section 4.2.1.1 of [RFC8955].  Type 8 component values SHOULD be
   encoded as a single octet (numeric_op len=00).

   In case of the presence of the ICMPv6 code component, only ICMPv6
   packets can match the entire Flow Specification.  The ICMPv6 code
   component, if present, never matches when the packet's upper-layer IP
   protocol value is not 58 (ICMPv6), if the packet is fragmented and
   this is not the first fragment, or if the system is unable to locate
   the transport header.  Different implementations may or may not be
   able to decode the transport header.

3.6.  Type 12 - Fragment

   Encoding:  <type (1 octet), [bitmask_op, bitmask]+>

   This defines a list of {bitmask_op, bitmask} pairs used to match
   specific IP fragments.

   This component uses the Bitmask Operator (bitmask_op) described in
   Section 4.2.1.2 of [RFC8955].  The Type 12 component bitmask MUST be
   encoded as a single octet bitmask (bitmask_op len=00).

                      0   1   2   3   4   5   6   7
                    +---+---+---+---+---+---+---+---+
                    | 0 | 0 | 0 | 0 |LF |FF |IsF| 0 |
                    +---+---+---+---+---+---+---+---+

                     Figure 1: Fragment Bitmask Operand

   Bitmask values:

   IsF:  Is a fragment other than the first -- match if IPv6 Fragment
         Header (Section 4.5 of [RFC8200]) Fragment Offset is not 0

   FF:   First fragment -- match if IPv6 Fragment Header (Section 4.5 of
         [RFC8200]) Fragment Offset is 0 AND M flag is 1

   LF:   Last fragment -- match if IPv6 Fragment Header (Section 4.5 of
         [RFC8200]) Fragment Offset is not 0 AND M flag is 0

   0:    MUST be set to 0 on NLRI encoding and MUST be ignored during
         decoding

3.7.  Type 13 - Flow Label (new)

   Encoding:  <type (1 octet), [numeric_op, value]+>

   This contains a list of {numeric_op, value} pairs that are used to
   match the 20-bit Flow Label IPv6 header field (Section 3 of
   [RFC8200]).

   This component uses the Numeric Operator (numeric_op) described in
   Section 4.2.1.1 of [RFC8955].  Type 13 component values SHOULD be
   encoded as 4-octet quantities (numeric_op len=10).

3.8.  Encoding Examples

3.8.1.  Example 1

   The following example demonstrates the prefix encoding for packets
   from ::1234:5678:9a00:0/64-104 to 2001:db8::/32 and upper-layer
   protocol tcp.

   +======+======================+=========================+==========+
   | len  | destination          | source                  | ul-proto |
   +======+======================+=========================+==========+
   | 0x12 | 01 20 00 20 01 0d bb | 02 68 40 12 34 56 78 9a | 03 81 06 |
   +------+----------------------+-------------------------+----------+

                                 Table 1

   Decoded:

   +=======+============+=================================+
   | Value |            |                                 |
   +=======+============+=================================+
   | 0x12  | length     | 18 octets (if len<240, 1 octet) |
   +-------+------------+---------------------------------+
   | 0x01  | type       | Type 1 - Dest. IPv6 Prefix      |
   +-------+------------+---------------------------------+
   | 0x20  | length     | 32 bits                         |
   +-------+------------+---------------------------------+
   | 0x00  | offset     | 0 bits                          |
   +-------+------------+---------------------------------+
   | 0x20  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0x01  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0x0d  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0xb8  | pattern    | (no padding needed)             |
   +-------+------------+---------------------------------+
   | 0x02  | type       | Type 2 - Source IPv6 Prefix     |
   +-------+------------+---------------------------------+
   | 0x68  | length     | 104 bits                        |
   +-------+------------+---------------------------------+
   | 0x40  | offset     | 64 bits                         |
   +-------+------------+---------------------------------+
   | 0x12  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0x34  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0x56  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0x78  | pattern    |                                 |
   +-------+------------+---------------------------------+
   | 0x9a  | pattern    | (no padding needed)             |
   +-------+------------+---------------------------------+
   | 0x03  | type       | Type 3 - Upper-Layer Protocol   |
   +-------+------------+---------------------------------+
   | 0x81  | numeric_op | end-of-list, value size=1, ==   |
   +-------+------------+---------------------------------+
   | 0x06  | value      | 06                              |
   +-------+------------+---------------------------------+

                           Table 2

   This constitutes an NLRI with an NLRI length of 18 octets.

   Padding is not needed either for the destination prefix pattern
   (length - offset = 32 bits) or for the source prefix pattern (length
   - offset = 40 bits), as both patterns end on an octet boundary.

3.8.2.  Example 2

   The following example demonstrates the prefix encoding for all
   packets from ::1234:5678:9a00:0/65-104 to 2001:db8::/32.

   +========+======================+=========================+
   | length | destination          | source                  |
   +========+======================+=========================+
   | 0x0f   | 01 20 00 20 01 0d b8 | 02 68 41 24 68 ac f1 34 |
   +--------+----------------------+-------------------------+

                             Table 3

   Decoded:

   +=======+=============+=================================+
   | Value |             |                                 |
   +=======+=============+=================================+
   | 0x0f  | length      | 15 octets (if len<240, 1 octet) |
   +-------+-------------+---------------------------------+
   | 0x01  | type        | Type 1 - Dest. IPv6 Prefix      |
   +-------+-------------+---------------------------------+
   | 0x20  | length      | 32 bits                         |
   +-------+-------------+---------------------------------+
   | 0x00  | offset      | 0 bits                          |
   +-------+-------------+---------------------------------+
   | 0x20  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0x01  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0x0d  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0xb8  | pattern     | (no padding needed)             |
   +-------+-------------+---------------------------------+
   | 0x02  | type        | Type 2 - Source IPv6 Prefix     |
   +-------+-------------+---------------------------------+
   | 0x68  | length      | 104 bits                        |
   +-------+-------------+---------------------------------+
   | 0x41  | offset      | 65 bits                         |
   +-------+-------------+---------------------------------+
   | 0x24  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0x68  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0xac  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0xf1  | pattern     |                                 |
   +-------+-------------+---------------------------------+
   | 0x34  | pattern/pad | (contains 1 bit of padding)     |
   +-------+-------------+---------------------------------+

                            Table 4

   This constitutes an NLRI with an NLRI length of 15 octets.

   The source prefix pattern is 104 - 65 = 39 bits in length.  After the
   pattern, one bit of padding needs to be added so that the component
   ends on an octet boundary.  However, only the first 39 bits are
   actually used for bitwise pattern matching, starting with a 65-bit
   offset from the topmost bit of the address.

4.  Ordering of Flow Specifications

   The definition for the order of traffic filtering rules from
   Section 5.1 of [RFC8955] is reused with new consideration for the
   IPv6 prefix offset.  As long as the offsets are equal, the comparison
   is the same, retaining longest-prefix-match semantics.  If the
   offsets are not equal, the lowest offset has precedence, as this Flow
   Specification matches the most significant bit.

   The code in Appendix A shows a Python3 implementation of the
   resulting comparison algorithm.  The full code was tested with Python
   3.7.2 and can be obtained at <https://github.com/stoffi92/draft-ietf-
   idr-flow-spec-v6/tree/master/flowspec-cmp>.

5.  Validation Procedure

   The validation procedure is the same as specified in Section 6 of
   [RFC8955] with the exception that item a) of the validation procedure
   should now read as follows:

   |  a)  A destination prefix component with offset=0 is embedded in
   |      the Flow Specification

6.  IPv6 Traffic Filtering Action Changes

   Traffic Filtering Actions from Section 7 of [RFC8955] can also be
   applied to IPv6 Flow Specifications.  To allow an IPv6-Address-
   Specific Route-Target, a new Traffic Filtering Action IPv6-Address-
   Specific Extended Community is specified in Section 6.1 below.

6.1.  Redirect IPv6 (rt-redirect-ipv6) Type 0x000d

   The redirect IPv6-Address-Specific Extended Community allows the
   traffic to be redirected to a VRF routing instance that lists the
   specified IPv6-Address-Specific Route-Target in its import policy.
   If several local instances match this criteria, the choice between
   them is a local matter (for example, the instance with the lowest
   Route Distinguisher value can be elected).

   This IPv6-Address-Specific Extended Community uses the same encoding
   as the IPv6-Address-Specific Route-Target Extended Community
   (Section 2 of [RFC5701]) with the Type value always 0x000d.

   The Local Administrator subfield contains a number from a numbering
   space that is administered by the organization to which the IP
   address carried in the Global Administrator subfield has been
   assigned by an appropriate authority.

   Interferes with: All BGP Flow Specification redirect Traffic
   Filtering Actions (with itself and those specified in Section 7.4 of
   [RFC8955]).

7.  Security Considerations

   This document extends the functionality in [RFC8955] to be applicable
   to IPv6 data packets.  The same security considerations from
   [RFC8955] now also apply to IPv6 networks.

   [RFC7112] describes the impact of oversized IPv6 header chains when
   trying to match on the transport header; Section 4.5 of [RFC8200]
   also requires that the first fragment must include the upper-layer
   header, but there could be wrongly formatted packets not respecting
   [RFC8200].  IPv6 Flow Specification component Type 3 (Section 3.3)
   will not be enforced for those illegal packets.  Moreover, there are
   hardware limitations in several routers (Section 1 of [RFC8883]) that
   may make it impossible to enforce a policy signaled by a Type 3 Flow
   Specification component or Flow Specification components that match
   on upper-layer properties of the packet.

8.  IANA Considerations

   This section complies with [RFC7153].

8.1.  Flow Spec IPv6 Component Types

   IANA has created and maintains a registry entitled "Flow Spec
   Component Types".  IANA has added this document as a reference for
   that registry.  Furthermore, the registry has been updated to also
   contain the IPv6 Flow Specification Component Types as described
   below.  The registration procedure remains unchanged.

8.1.1.  Registry Template

   Type Value:  contains the assigned Flow Specification component type
                value

   IPv4 Name:   contains the associated IPv4 Flow Specification
                component name as specified in [RFC8955]

   IPv6 Name:   contains the associated IPv6 Flow Specification
                component name as specified in this document

   Reference:   contains references to the specifications

8.1.2.  Registry Contents

   Type Value:  0
   IPv4 Name:   Reserved
   IPv6 Name:   Reserved
   Reference:   [RFC8955], RFC 8956

   Type Value:  1
   IPv4 Name:   Destination Prefix
   IPv6 Name:   Destination IPv6 Prefix
   Reference:   [RFC8955], RFC 8956

   Type Value:  2
   IPv4 Name:   Source Prefix
   IPv6 Name:   Source IPv6 Prefix
   Reference:   [RFC8955], RFC 8956

   Type Value:  3
   IPv4 Name:   IP Protocol
   IPv6 Name:   Upper-Layer Protocol
   Reference:   [RFC8955], RFC 8956

   Type Value:  4
   IPv4 Name:   Port
   IPv6 Name:   Port
   Reference:   [RFC8955], RFC 8956

   Type Value:  5
   IPv4 Name:   Destination Port
   IPv6 Name:   Destination Port
   Reference:   [RFC8955], RFC 8956

   Type Value:  6
   IPv4 Name:   Source Port
   IPv6 Name:   Source Port
   Reference:   [RFC8955], RFC 8956

   Type Value:  7
   IPv4 Name:   ICMP Type
   IPv6 Name:   ICMPv6 Type
   Reference:   [RFC8955], RFC 8956

   Type Value:  8
   IPv4 Name:   ICMP Code
   IPv6 Name:   ICMPv6 Code
   Reference:   [RFC8955], RFC 8956

   Type Value:  9
   IPv4 Name:   TCP Flags
   IPv6 Name:   TCP Flags
   Reference:   [RFC8955], RFC 8956

   Type Value:  10
   IPv4 Name:   Packet Length
   IPv6 Name:   Packet Length
   Reference:   [RFC8955], RFC 8956

   Type Value:  11
   IPv4 Name:   DSCP
   IPv6 Name:   DSCP
   Reference:   [RFC8955], RFC 8956

   Type Value:  12
   IPv4 Name:   Fragment
   IPv6 Name:   Fragment
   Reference:   [RFC8955], RFC 8956

   Type Value:  13
   IPv4 Name:   Unassigned
   IPv6 Name:   Flow Label
   Reference:   RFC 8956

   Type Value:  14-254
   IPv4 Name:   Unassigned
   IPv6 Name:   Unassigned

   Type Value:  255
   IPv4 Name:   Reserved
   IPv6 Name:   Reserved
   Reference:   [RFC8955], RFC 8956

8.2.  IPv6-Address-Specific Extended Community Flow Spec IPv6 Actions

   IANA maintains a registry entitled "Transitive IPv6-Address-Specific
   Extended Community Types".  For the purpose of this work, IANA has
   assigned a new value:

      +============+===================================+===========+
      | Type Value | Name                              | Reference |
      +============+===================================+===========+
      | 0x000d     | Flow spec rt-redirect-ipv6 format | RFC 8956  |
      +------------+-----------------------------------+-----------+

            Table 5: Transitive IPv6-Address-Specific Extended
                         Community Types Registry

9.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5701]  Rekhter, Y., "IPv6 Address Specific BGP Extended Community
              Attribute", RFC 5701, DOI 10.17487/RFC5701, November 2009,
              <https://www.rfc-editor.org/info/rfc5701>.

   [RFC7112]  Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", RFC 7112,
              DOI 10.17487/RFC7112, January 2014,
              <https://www.rfc-editor.org/info/rfc7112>.

   [RFC7153]  Rosen, E. and Y. Rekhter, "IANA Registries for BGP
              Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
              March 2014, <https://www.rfc-editor.org/info/rfc7153>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8883]  Herbert, T., "ICMPv6 Errors for Discarding Packets Due to
              Processing Limits", RFC 8883, DOI 10.17487/RFC8883,
              September 2020, <https://www.rfc-editor.org/info/rfc8883>.

   [RFC8955]  Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
              Bacher, "Dissemination of Flow Specification Rules",
              RFC 8955, DOI 10.17487/RFC8955, December 2020,
              <https://www.rfc-editor.org/info/rfc8955>.

Appendix A.  Example Python Code: flow_rule_cmp_v6

   <CODE BEGINS>
   """
   Copyright (c) 2020 IETF Trust and the persons identified as authors
   of the code. All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, is permitted pursuant to, and subject to the license
   terms contained in, the Simplified BSD License set forth in Section
   4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info).
   """

   import itertools
   import collections
   import ipaddress

   EQUAL = 0
   A_HAS_PRECEDENCE = 1
   B_HAS_PRECEDENCE = 2
   IP_DESTINATION = 1
   IP_SOURCE = 2

   FS_component = collections.namedtuple('FS_component',
                                         'component_type value')

   class FS_IPv6_prefix_component:
       def __init__(self, prefix, offset=0,
                    component_type=IP_DESTINATION):
           self.offset = offset
           self.component_type = component_type
           # make sure if offset != 0 that none of the
           # first offset bits are set in the prefix
           self.value = prefix
           if offset != 0:
               i = ipaddress.IPv6Interface(
                   (self.value.network_address, offset))
               if i.network.network_address != \
                   ipaddress.ip_address('0::0'):
                   raise ValueError('Bits set in the offset')

   class FS_nlri(object):
       """
       FS_nlri class implementation that allows sorting.

       By calling .sort() on an array of FS_nlri objects these
       will be sorted according to the flow_rule_cmp algorithm.

       Example:
       nlri = [ FS_nlri(components=[
                FS_component(component_type=4,
                             value=bytearray([0,1,2,3,4,5,6])),
                ]),
                FS_nlri(components=[
                FS_component(component_type=5,
                             value=bytearray([0,1,2,3,4,5,6])),
                FS_component(component_type=6,
                             value=bytearray([0,1,2,3,4,5,6])),
                ]),
              ]
       nlri.sort() # sorts the array according to the algorithm
       """
       def __init__(self, components = None):
           """
           components: list of type FS_component
           """
           self.components = components

       def __lt__(self, other):
           # use the below algorithm for sorting
           result = flow_rule_cmp_v6(self, other)
           if result == B_HAS_PRECEDENCE:
               return True
           else:
               return False

   def flow_rule_cmp_v6(a, b):
       """
       Implementation of the flowspec sorting algorithm in
       RFC 8956.
       """
       for comp_a, comp_b in itertools.zip_longest(a.components,
                                              b.components):
           # If a component type does not exist in one rule
           # this rule has lower precedence
           if not comp_a:
               return B_HAS_PRECEDENCE
           if not comp_b:
               return A_HAS_PRECEDENCE
           # Higher precedence for lower component type
           if comp_a.component_type < comp_b.component_type:
               return A_HAS_PRECEDENCE
           if comp_a.component_type > comp_b.component_type:
               return B_HAS_PRECEDENCE
           # component types are equal -> type-specific comparison
           if comp_a.component_type in (IP_DESTINATION, IP_SOURCE):
               if comp_a.offset < comp_b.offset:
                   return A_HAS_PRECEDENCE
               if comp_a.offset > comp_b.offset:
                   return B_HAS_PRECEDENCE
               # both components have the same offset
               # assuming comp_a.value, comp_b.value of type
               # ipaddress.IPv6Network
               # and the offset bits are reset to 0 (since they are
               # not represented in the NLRI)
               if comp_a.value.overlaps(comp_b.value):
                   # longest prefixlen has precedence
                   if comp_a.value.prefixlen > \
                       comp_b.value.prefixlen:
                       return A_HAS_PRECEDENCE
                   if comp_a.value.prefixlen < \
                       comp_b.value.prefixlen:
                       return B_HAS_PRECEDENCE
                   # components equal -> continue with next
                   # component
               elif comp_a.value > comp_b.value:
                   return B_HAS_PRECEDENCE
               elif comp_a.value < comp_b.value:
                   return A_HAS_PRECEDENCE
           else:
               # assuming comp_a.value, comp_b.value of type
               # bytearray
               if len(comp_a.value) == len(comp_b.value):
                   if comp_a.value > comp_b.value:
                       return B_HAS_PRECEDENCE
                   if comp_a.value < comp_b.value:
                       return A_HAS_PRECEDENCE
                   # components equal -> continue with next
                   # component
               else:
                   common = min(len(comp_a.value),
                                len(comp_b.value))
                   if comp_a.value[:common] > \
                       comp_b.value[:common]:
                       return B_HAS_PRECEDENCE
                   elif comp_a.value[:common] < \
                         comp_b.value[:common]:
                       return A_HAS_PRECEDENCE
                   # the first common bytes match
                   elif len(comp_a.value) > len(comp_b.value):
                       return A_HAS_PRECEDENCE
                   else:
                       return B_HAS_PRECEDENCE
       return EQUAL
   <CODE ENDS>

Acknowledgments

   The authors would like to thank Pedro Marques, Hannes Gredler, Bruno
   Rijsman, Brian Carpenter, and Thomas Mangin for their valuable input.

Contributors

   Danny McPherson
   Verisign, Inc.

   Email: dmcpherson@verisign.com

   Burjiz Pithawala
   Individual

   Email: burjizp@gmail.com

   Andy Karch
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134
   United States of America

   Email: akarch@cisco.com

Authors' Addresses

   Christoph Loibl (editor)
   next layer Telekom GmbH
   Mariahilfer Guertel 37/7
   1150 Vienna
   Austria

   Phone: +43 664 1176414
   Email: cl@tix.at

   Robert Raszuk (editor)
   NTT Network Innovations
   940 Stewart Dr
   Sunnyvale, CA 94085
   United States of America

   Email: robert@raszuk.net

   Susan Hares (editor)
   Huawei
   7453 Hickory Hill
   Saline, MI 48176
   United States of America

   Email: shares@ndzh.com